In the field of thermal ink jet printing, it is known to provide metal orifice plates, such as those fabricated of electroplated nickel, in combination with thin film resistor (TFR) heater substrates in the construction of thermal ink jet printheads. The art of thermal ink jet printhead construction is relatively well developed and is disclosed, for example, in some detail in the Hewlett Packard Journal, Vol. 36, No. 5, May 1985 incorporated herein by reference. Electroformed nickel orifice plate structures are also disclosed in copending application Ser. No. 801,169 of C. S. Chan et al entitled "Improved Barrier Layer and Orifice Plate for Thermal Ink Jet Printhead Assembly and Method of Manufacture", filed Nov. 22, 1985 and assigned to the present assignee and also incorporated herein by reference.
In this copending application of Chan et al, there is disclosed and claimed an improved convergent nozzle geometry which greatly reduces the problem of gulping of the ink during an ink ejection operation and minimizes the attendant problems of cavitation wear and operational inefficiency produced by this gulping. However, it has been found that when employing the process of this copending Chan et al application, there is a trade-off between orifice packing density and orifice plate thickness. That is, for a given minimum exit diameter of the ink ejection orifice, a closer packing density or center-to-center spacing of the orifices would require a reduction in electroplating or electroforming thickness and structural integrity. This trade-off is usually undesirable, since a reduced orifice plate thickness will also mean a corresponding decrease in orifice bore thickness and this will in turn increase trajectory error of the ink drops. It is the solution to this latter problem to which the present invention is directed.